The primary sensory areas of neocortex perform complex transformations of the information flowing into them. In the primary somatosensory (SmI) area of the rat, many of these operations are carried out by discrete, repeating units of interconnected neurons, each of which can be visually identified by a barrel-shaped aggregate of small cells in cortical layer IV. Under pathological conditions, such as a decrease in synaptic inhibition, the normal activity of a local neocortical circuit can be usurped by large, synchronized bursts of epileptic excitation in all of its neurons. The proposed research will investigate the morphological and physiological properties of the neurons and their synaptic connections within rat barrel cortex. A method has been devised for visualizing individual barrels in living cortical slices in vitro. Intracellular recordings plus dye injections will be used to correlate membrane properties with the soma-dendritic and axonal branching characteristics of neurons. Electrophysiological techniques and axonal labelling methods, carried out in the slices and in intact animals, will help to delineate interlaminar and interbarrel connections. A combination of these methods and immunocytochemical identification will characterize the spatial organization of local inhibitory circuits that use the neurotransmitter gamma- aminobutyric acid. The mechanisms of synchronized epileptic discharge will also be studied. Specifically, the hypothesis that a subpopulation of middle layer bursting cells initiates and disperses synchronized activity will be examined. This interdisciplinary approach should provide a uniquely detailed view of the functional properties of a local neocortical area. The information obtained will contribute to the formulation of theories of cortical information processing, as well as suggest mechanisms for the genesis and control of focal seizures.